Somewhere between R7 and R8, the Appointment form was changed so that the body field is now located within a tab table. Unfortunately, the tab key is used for navigating from one tab to the next, even if only one tab table row is visible.
If you are simply trying to indent a PPH, you can use the F8 key, or Shift+F8 to outdent.
If you are using the tab key for other formatting purposes, you should know that extraneous spaces, such as those added using the Tab key will most likely be lost when sending a meeting invitations or email to an external Internet email address. Instead of the Tab key, you should consider using a table.
If you decide the Tab key is what you really want to use, there are 3 ways this can be fixed:
Fix 1. Revert back to the R7 mail template. You'll want to consult with your Notes admin before doing this, as there could be some loss of data.
Fix 2. Open the DB in Designer. Rename or delete the existing Appointment form, then copy the R7 form from the R7 template to your mail database. Make sure you prohibit design refresh for the R7 appointment form, or it will be replaced the next time the design is refreshed. Have no idea if there are any shared design elements in the R7 form that are no longer available in R8... you'll find out soon enough, and if necessary can revert back to the R8 Appointment form by removing the prohibit design refresh check mark, then refreshing the design.
Fix 3. Open the Appointment form in designer and move the body field to the bottom of the form, outside the tab table. You'll still see the tab table, but you will now be able to use the Tab key as expected. With this option, you will also want to prohibit design refresh.